In recent years, a technique for forming a thin film transistor (TFT) by using a thin semiconductor film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been applied to a wide range of electronic devices like integrated circuits (ICs) and electro-optical devices, and development of thin film transistors as switching elements in image display devices, in particular has been hastened.
Metal oxides have been known as materials having semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics include a tungsten oxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such metal oxides having semiconductor characteristics is already known (for example, see Patent Documents 1 and 2).
A liquid crystal display device is known as an example of image display devices. Compared with passive matrix liquid crystal display devices, high-definition images can be obtained with active matrix liquid crystal display devices; therefore, the active matrix liquid crystal display devices have become widely used. In the active matrix liquid crystal display device, pixel electrodes arranged in matrix are driven to display an image on a screen. Specifically, a video voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, and thus, a liquid crystal layer between the pixel electrode and the counter electrode is modulated optically. Light is modulated so that an image is displayed and an observer recognizes the image.
An image burn-in may be caused when a voltage is applied to a liquid crystal only in one direction all the time. Thus, an AC drive is employed in which a voltage applied to a liquid crystal layer is periodically inverted. In an AC drive, a voltage applied to a liquid crystal is to be periodically inverted; for example, the polarity of a video voltage based on a voltage of a counter electrode (a common electric potential) is inverted every frame.
In addition, an example of an AC drive in which a counter electrode is divided into two counter electrodes and different potentials are applied thereto in a pixel portion is described in Patent Document 3.